1. Field of the Invention
This invention relates to an automatic transmission for automobiles utilizing a continuously variable transmission (hereinafter called a CVT) and a method of controlling the same.
2. Description of the Prior Art
The speed ratio in conventional automatic transmissions for automobiles is changed stepwise from the first to fourth steps, for example, and the change of step is determined by the rotational speed of the output shaft of the transmission, i.e. by the vehicle speed and the throttle (valve) position of an intake system to provide a predetermined hysterisis between shift-up and shift-down. Also, when the first speed is changed to the second one at a predetermined rotational speed of an output shaft within the low range, the first speed is thereafter maintained irrespective of the rotational speed of the output shaft. By such control of the change of step, sufficient driveability is assured over a wide running range of the engine. However the fact that the speed ratio in the respective speed change step region is fixed, the provision of the hysteresis, etc. are all disadvantageous to fuel consumption, and the low efficiency of power transmission of a hydraulic torque converter used together with a stepped automatic transmission results in a reduction in fuel consumption. Further, the occurrence of impulses in the speed change also provides a drawback.
While in general methods of controlling a CVT, the speed ratio e of the CVT is controlled so that (1) the actual engine speed Ne provides the desired engine speed Ne', (2) the amount of change .DELTA.Ne per a predetermined time of the engine speed Ne in the speed change, i.e. the change of the speed ratio e is controlled to be a desired value or (3) the speed ratio e provides a desired speed ratio e', the rate of speed change, i.e. the differential value de/dt of the speed change with respect to time is not limited in any of these controlling methods. Since the transmission loss of the CVT in the speed change of the CVT is increased, the specific fuel consumption in the speed change is degraded by such controlling methods.